Current work can be divided into three main divisions. 1. The mechanism by which circulating soluble fibrin monomer is precipitated as fibrin in organs. Specifically, the role of granulocyte, platelet, or endothelial constituents in this process. 2. The mechanism by which high molecular weight fibrinogen is converted into its lower molecular weight derivative in the blood. The study includes an investigation of these two major fibrinogen fractions in healthy subjects and in certain vascular diseases. 3. A study of the action of vascular activator on fibrinogen versus fibrin degradation. The findings will be compared with those of streptokinase and urokinase.